As a work machine, there has been known a hybrid work machine such as an excavator in which an engine drives a generator motor and a hydraulic pump, whereby an electric motor for an upper rotary body, in which a cab and the like are provided, is driven by electric power generated by the generator motor while a hydraulic actuator for a working equipment and a hydraulic motor for a traveling device are driven by a pressure oil from the hydraulic pump. The electric power generated by the generator motor is charged in a capacitor serving as an electric storage device and is outputted from the capacitor to the electric motor through an inverter.
In some hybrid work machines, charge release of an electric storage device (i.e., capacitor) is performed before measurement of the capacitance of the electric storage device (see, for instance, Patent Literature 1). According to Patent Literature 1, prior to capacitance measurement, charge release (discharge) of the capacitor is preferably performed to reduce a capacitor voltage to a preset level for the purpose of comparison with previous measurement data.
Further, for capacitance measurement, an engine is typically driven and a work mode is set at a power mode or the like, thereby driving the engine and a generator motor at a stable speed to stably generate electric power and thus to stably and accurately measure the capacitance.